According to Irvin Gottlieb in his work titled “Electric Motors and Control Techniques”, Second Edition Page 72 McGraw Hill 1994 U.S.A., the induction motor can be understood as a rotary transformer, this due to the effect present in large capacity or very large transformers where the design of these present special attention to the mechanic repulsion force which exists between the primary and secondary coils; thus this repulsion phenomenon between coils in some way gives life to the asynchronous squirrel cage motor; the “primary” being the stator and the “secondary” in short circuit the rotor, this being an advantageous design compared to the synchronous machines (motors) as they do not require permanent magnets, “carbons”, brushes or switches.
The squirrel cage motors, above all the tri phase motors of this kind, are fairly common for diverse industrial applications, due to their construction simplicity, robust design and efficiency among other virtues. Thus this type of motors is desirable for other applications, such as household washers or refrigerators, where in the example with the washer it has the inconvenience of the velocity control, since these motors work very well at a constant velocity, it being rather difficult to control the velocity of these. A first effort is described in the already referenced bibliography by Gottlieb in pages 91 and 92, where it explains a variation in an asynchronous squirrel cage motor where the coils of the rotor are landed or connected to some sliding rings, and these in turn, are connected to some rheostats. Upon doing this the rotor velocity can then be controlled by increasing or decreasing the resistance in the rheostats, as these can cause an increase or a decrease in the dephasing or slipping between the velocity of the magnetic field in the stator and the mechanical velocity of the rotor. This type of motor construction turns at a certain undesired point upon knowing that the sliding rings require brushes or carbons, themselves elements of wearing which also allow the flow of current; this in addition to increasing the cost and complexity of the motor construction, thereby increasing maintenance cost.
Another effort to control the velocity of a squirrel cage motor is found in US document 6316895 B1 by Ramarathnam, where the methodology of “Pulse Width Modulation” (PWM) is used where the quotient between the voltage and the frequency is maintained at a constant and through this, it is possible to maintain constant torque, allowing for velocity variation with lower voltages than the motor work voltage, with such luck that with the mere frequency variation the motor velocity is controlled. This document also explains an interesting variation of the PWMs, by using Spatial Vectors, thus the theory explained in the mentioned document, develops a logic named Space Vector Pulse Width Modulation (“SVPWM”). Thus using these techniques can successfully result in the programming of an electric control so that it may itself for certain periods of time can turn on and off the switches with such luck that three sinusoidal signals are obtained, dephased between themselves, which feed the coils of the induction motor stator. Initially this solution has an interesting result but overlooks the motor slipping control, because it itself, upon feeling a frequency variation will tend to increase or decrease its velocity in relation to the frequency fed, but if the motor has coupled to its axis a large mechanical load which causes a great inertia, the rotor will not be able to follow the magnetic field frequency induced by the stator, thus increasing its dephasing or slip between the angular velocity or magnetic field frequency and the mechanical velocity of the rotor shaft. Said control ends up being important knowing that there is no point to keep on increasing the stator frequency if the rotor cannot follow it, it would only cause overheating of the motor and even reach a frequency in which there is no induction on the rotor and it can cause it to stop.
Thus with the intention of resolving these and other inconveniences in addition to providing a precise, reliable and low cost control, among others, a solution is proposed, objective of the present invention.